Differential Phased Shift Keying (DPSK, also known as Differential Binary Phased Shift Keying DBPSK) and Differential Quadrature Phased Shift Keying (DQPSK) are very attractive modulation formats for optical data transmission. When used with a balanced direct detection receiver, DPSK outperforms conventional on-off keying receiver sensitivity by approximately 3 dB. Dense Wavelength Division Multiplexing (DWDM) transmission up to 10,000 km has been demonstrated at 40 Gbit/s using DPSK modulation format. DQPSK has a symbol rate that is half of the data rate. For example, for a 43 Gb/s data rate, a 21.5 gigasymbol per second rate is used. As a result, DQPSK has a narrower spectral bandwidth, greater chromatic dispersion tolerance and greater tolerance with respect to polarization mode dispersion (PMD) than traditional formats or DPSK.
DPSK and DQPSK can be non-return-to-zero NRZ-type modulated or, if a return-to-zero (RZ) pulse carver is added to the transmitter, RZ-type modulated. RZ-type usually outperforms NRZ-type formats in performance based on optical signal-to-noise ratio (OSNR) sensitivity and robustness with respect to nonlinearity.
DPSK and DQPSK modulation formats require rather complicated transmitters and receivers. FIGS. 1-3 illustrate transmitters 20, 24 and receivers 22, 26 for DPSK and DQPSK modulation formats. For proper operation, transmitters 20, 24 and receivers 22, 26 should be properly tuned. For commercial use, control loops are implemented to contribute to proper tuning and to maintain proper conditions in the operation of transmitters 20, 24 and receivers 22, 26.
Optical receivers include asymmetric Mach-Zehnder interferometers, also commonly referred to as delay interferometers (DIs), which act as optical DPSK/DQPSK demodulators, and balanced photodetectors. The time delay between the two arms of the interferometer is approximately an integer number of the time symbol slots of the optical DPSK/DQPSK data signal:Δt=n T  (1)
where n=1, 2, . . . T=1/B is the symbol slot, and B is the symbol bit rate. An optical demodulator converts the DPSK/DQPSK phase-modulated signal into an amplitude-modulated optical signal at one output and into an inverted amplitude-modulated optical signal at the other output. These signals are detected with a balanced detector that consists of two high-speed detectors such as PIN diodes 23, 27 illustrated in receivers 22, 26. The outputs of the detectors are electrically subtracted from each other, and the resultant electrical signal is sent to data recovery circuits.
For a DPSK receiver to operate properly, the asymmetric Mach-Zehnder interferometer or DI should be accurately phased-tuned or biased. DPSK receiver 22 uses one DI 21, and the optimum phase bias between the two arms is π or 0. DQPSK receiver 26 uses two DIs 25, 28. One DI is tuned to a +π/4 bias, and the other DI is tuned to a −π/4 bias.
The modulators in DPSK and DQPSK transmitters 20, 24 also should be properly biased. Typical parameters known to influence modulator control include modulator biases, relative timing between the RZ pulse carving by the RZ modulator and the data modulation and a time alignment between the two data streams (for DQPSK transmitter 24). DQPSK transmitter 24 consists of two parallel Mach-Zehnder (MZ) modulators 29. The optical signals from modulators 29 are combined to produce an RZ DQPSK output. The proper relative optical phase or bias between these signals of π/2 is set by a phase shifter 32. In known systems, phase shifter 32 operates on the basis of optical power feedback.